Electrostatic-piezoelectric transducer

ABSTRACT

An electrostatic type electroacoustic transducer having improved acoustic characteristics is composed of a piezoelectric polymertype oscillator having a stretching-type piezoelectric effect, such as a polarized polyvinylidene fluoride film having electrodes vacuum-coated on both surfaces and at least one fixed electrode disposed facing the oscillator and spaced therefrom.

m 3,89%9198 if 7 W 3 a a united bran I t t I [111 3,894,198 Murayama et a1. July 8, 1975 [54] ELECTROSTATIC-PIEZOELECTRIC 2,478,223 8/1949 Argabrite 179/110 F TRANSDUCER 3,646,280 2/1972 Tamura et al....

3,646,413 2/1972 Oomen 3lO/8.6 [75] Inventors: Haohiro Murayama; Takao Oikawa;

Kenichi Nakamura, all of Iwaki, FOREIGN PATENTS OR APPLICATlONS Japan 2,000,770 9/1969 France .0 310/33 [73] Assignee: Kureha Kagaku Kogyo Kabushiki Tokyo Japan Primary Examiner-Kathleen H. Claffy [22] Filed; Nov, 6, 1972 Assistant ExaminerGeorge G. Stellar Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, [21] Appl. No.. 303,894 Zinn & Macpeak [30] Foreign Application Priority Data Nov. 4, 1971 Japan 46-87854 [57] ABSTRACT An electrostatic type electroacoustic transducer having improved acoustic characteristics is composed of a piezoelectric polymertype oscillator having a stretch- [52] US. Cl...... 179/110 A; 179/111 R; 179/111 E; 310/82; 310/85; 310/95 4 E l Search 8:4 a mg-type plezoelectrlc effect, such as a polarized poly- 179H06 310/86 8 3 317/144f vinylidene fluoride film having electrodes vacuum- 5 1 coated on both surfaces and at least one fixed electrode disposed facing the oscillator and spaced there- [56] References Cited from UNITED STATES PATEN 4 Claims, 10 Drawing Figures 1,889,748 12/1932 Gruschke 179/111 R PATENTEDJUL 8 ms SHEET PRIOR ART PRIOR ART PRIOR ART HUI ELECTROSTATIC-PIEZOELECTRIC TRANSDUCER BACKGROUND OF THE INVENTION l. Field of the Invention:

The present invention relates to an electrostatic type electroacoustic transducer having as the oscillator a piezoelectric polymer showing a piezoelectric effect of developing a stretching motion in the direction perpendicular to the electric axis of the piezoelectric element.

2. Description of the Prior Art:

Electrostatic type electroacoustic transducers having as the oscillator a piezoelectric substance composed of a polymer such as a vinylidene fluoride resin as illus trated in FIG. I of the accompanying drawings are well known. FIG. I is a schematic view showing the principle of the conventional piezoelectric type electroacoustic transducers using a piezoelectric polymer as the oscillator. In the figure, an oscillator film l composed of a piezoelectric substance of a polymer such as a vinylidene fluoride resin having stretching type piezoelectricity has electrodes 2 and 2 at both surfaces thereof. The electrodes are formed by vacuum depositing or attaching a conductor.

The stretching type piezoelectric effect referred to above is explained by referring to FIG. 2 of the accompanying drawings. That is, when an electric field is applied to a piezoelectric element A in the direction of the electric axis B of the element, the piezoelectric element A develops an expanding or contracting movement in the direction C perpendicular to the electron axis direction B. In the electroacoustic transducer as shown in FIG. I, when a signalfrom an electric system is applied to the electrodes 2 and 2', a stretching motion induced by the aforesaid piezoelectric effect occurs as shown by a dotted line in the FIG. 1, whereby the electroacoustic conversion is effected. The electroacoustic transducer as mentioned above is sufficient for some practical purposes.

However, when a piezoelectric polymer film which acts as an oscillator is used in such an electroacoustic transducer. the piezoelectric film is required to show good acoustic characteristics in addition to acting as the oscillator and further it is desirable that the piezoelectric film give a larger sound output so that the electroacoustic transducer can be easily made.

Furthermore, when a stretching type piezoelectric polymer film is utilized as an oscillator for speakers, and the like, the polymer film develops a stretching movement by the application of an electric field to both surfaces of the polymer film, but when the polymer film is stretched in a plane, only the tension of the polymer film changes by the stretching movement and the film does not oscillate. Accordingly, it is required to provide a curvature to the polymer film. In order to stretch the polymer film while providing a curvature thereto, the film must be supported. resulting in the oscillations of the film being hindered by the support and the sound thus distorted. Thus, the use of a support for a curved film is inevitably accompanied with the problem that the planning and manufacturing of acoustic transducers becomes. in general. complicated.

An example. of the conventional electrostaticor capacitor-type electroacoustic transducer is illustrated in FIG. 3, which shows the principle ofa cupacitortype electroacoustic transducer using a fixed electret electrode. In FIG. 3 an oscillator 3 is composed of an oscillator film 4 and an electrode 5 attached thereto and a fixed electret electrode 6 is composed of an electret 7 and a back plate 8 vacuum-deposited or attached thereto. When the surface of the electret 7 facing the oscillator 3 is, as shown in the FIG. 3, charged positively by the polarization potential of the electret, the surface of the oscillator 3 is charged negatively.

In this case, the back plate 8 of the fixed electrode has been connected to the one of the terminals of the secondary winding of a transformer 15, while the electrode 5 of the oscillator has been connected to the other of the terminals of the same secondary winding of the transformer.

If an ordinary fixed electrode is used in a capacitortype electroacoustic transducer, a dc. bias potential is required but in case of the electret type fixed electrode as mentioned above, the application of a bias potential is unnecessary. When an alternating current signal is applied to the primary winding of the transformer 15 through a signal source 16, the electrostatic charge on the fixed electrode 6 changes to provide an electrostatic attractive force or an electrostatic repulsive force between the fixed electrode 6 and the oscillator 3, which results in causing the oscillation of the oscillator 3 corresponding to the a.c. potential thus applied.

SUMMARY OF THE INVENTION The inventors have investigated techniques for improving the sound output or sensitivity of electroacoustic transducers and as the result it has been discovered that an electroacoustic transducer having excellent characteristics can be obtained by combining the characteristics of a capacitor type electrostatic; electroacoustic transducer and the characteristics of a piezoelectric type electroacoustic transducer.

In the present invention, an electroacoustic transducer of the abovementioned electrostaticor capacitor-type is constructed by employing a piezoelectric polymer film, the film having electrodes on both surfaces thereof asthe oscillator film.

In regard to FIG. 1 the principle was stated that the piezoelectric type electroacoustic transducer using a piezoelectric polymer film as the oscillator are old. In the present invention, by utilizing the oscillation caused by the piezoelectric polymer film type oscillator and the oscillation caused by the aforesaid capacitor-type electroacoustic transducer in a superposed relationship, larger oscillations have been obtained.

It is known that the polarity of the piezoelectric polymer film can be determined by the polarity of a dc. electric field applied to the polymer film under heating for providing piezoelectricity to the polymer film. Therefore. it is possible to know quite easily whether a piezoelectric polymer film prepared by the abovementioned method expands or contracts when the piezoelectric polymer film is used as the oscillator film for an electroacoustic transducer and a positive or negative electric field is applied thereto.

Thus, by so constructing or planning that the oscillation, as a capacitor-type electroacoustic transducer due to an a.c. electric field applied to a piezoelectric polymer film employed in the transducer as the oscillator, coincides with the oscillation as a piezoelectric-type electroacoustic transducer. a quite large or strong oscillation can be obtained. A

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a prior art piezoelectric transducer.

FIG. 2 illustrates the operation of the piezoelectric transducer shown in FIG. 1.

FIG. 3 is a prior art electrostatic transducer.

FIGS. 4, 4A and 4B show an electrostatic type electroacoustic transducer in accordance with the present invention.

FIG. 5 shows another embodiment of an electrostatic type electroacoustic transducer in accordance with the present invention.

FIGS. 6 and 7 show the equivalent electrical circuits of the embodiments in FIGS. 4A and B and FIG. 5.

FIG. 8 shows another embodiment of an electrostatic type electroacoustic transducer in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now. the principle of electroacoustic transducer of this invention will be explained more in detail by referring to FIG. 4, FIG. 4A and FIG. 48. FIG. 4 shows this transducer in a stationary unexcited state prior to the vibrating operation shown in FIGS. 4A and 4B and prior to supplying the electrical connections among the electrodes and the ac. input. In FIG. 4A, a fixed electrode 9 of the electroacoustic transducer of this invention, which is in an ac signal free state, is disposed facing an oscillator 12 consisting of an oscillator film 14 sandwiched between electrodes 13. The surface of the fixed electrode has been charged positively, while the surface of the oscillator 12 has been charged negatively in the state shown in the figure. The fixed electrode 9 is composed of an electret l0 and a back plate 11. If a bias electric source is used, the fixed electrode 9 may be composed of only a conductive plate without an electret.

As illustrated in FIG. 4A, the oscillator 12 has been charged negatively as mentioned above by the polarization potential on the electret 10 and thus the oscillator 12 has been deformed to the side of the fixed electrode by the attractive force caused by the static charges on the electret 10.

Then, when an alternating current signal is applied to the fixed electrode 9 from a signal source through a transformer 15, which may be omitted as the case may be, the charges on the fixed electrode change and thus the oscillator is vibrated by the electrostatic attractive and repulsive forces. In this case, the oscillator film 14 is made of a piezoelectric polymer film. the oscillator film itself oscillates by the piezoelectric effect in proportion to the change of the electrostatic charges.

In the embodiment illustrated in FIG. 48. when a positive potential is. for example. applied to the fixed electrode 9 as a signal. the oscillator 12, again comprising the film 14 sandwiched between electrodes 13, approaches the position shown by the dotted lines according to the principle of the electrostatic electroacoustic transducer mentioned above corresponding to a half period of the alternating current signal thus applied. Further if the polarity of the piezoelectric polymer film I4 is so selected that the oscillator expands by the increase of the negative charges on the oscillator on the side near the fixed electrode. the oscillator 12 further approaches the fixed electrode side and the deformation of the oscillator is further increased to the position shown by the solid lines as shown in FIG. 4B.

When an opposite electric field is applied to the system. the oscillator 12 moves to a largely deformed position opposite to the above by the repulsive force of the fixed electrode and the reduction of the curvature of the oscillator by the contraction of the piezoelectric polymer film 14. Accordingly, for effectively utilizing the change of the curvature by a contraction of the pi ezoelectric polymer film, the oscillator may be provided with some curvature even in its contracted state.

As indicated in FIGS. 4, 4A and 4B, the electrode 9 and one oscillator electrode 13 are to be connected to one side of the secondary winding of transformer I5, whereas the other oscillator electrode I3 is to be connected to the other terminal of the secondary winding.

Furthermore, when a mechanical oscillation is applied to the oscillator and an electric signal is produced for an output as in the case of a microphone. a large change in electric potential can be obtained by an electroacoustic transducer of the same structure based on the same principle.

In order to charge the oscillator. a dc. bias voltage may be applied to the oscillator, as in a general electrostatic type transducer, or an electret is disposed near the oscillator and the electric field thereof may be utilized. The latter system of using the electret is particularly convenient since this requires no transformer having an intermediate tap for increasing the voltages for the polarization potential and the input signal and at the same time dividing them to give opposite charges to both fixed electrodes disposed on opposite sides of the oscillator.

Moreover. besides the single-type electroacoustic transducer indicated above. the principle of this invention can apply to a push-pull type electroacoustic transducer as illustrated in FIG. 5 of the accompanying drawings.

As in the above example of this invention, an oscillator 12 in FIG. 5 is composed ofa piezoelectric polymer film and electrodes vacuum-coated or attached to both surfaces thereof. Two fixed electrodes 9 and 9 each comprise an electret and an electrode disposed at the opposite sides of the oscillator 12, with a proper interval and the fixed electrodes are connected across the terminals of the secondary winding ofa transformer 15. The electrodes of the oscillator 12 are also connected across the secondary winding of the transformer. A signal source 16 has been connected to the primary winding of the transformer IS.

The electrets of the fixed electrodes 9 and 9' have been charged preliminarily as indicated in FIG. 5 and when a signal is applied to the fixed electrodes from the signal source 16, the charge on the surface of one of the electrets is cancelled, while the charge on the other is increased.

By repeating such an action. the oscillator causes oscillation. Furthermore, the oscillator develops a bending or bowing movement by the piezoelectric effect due to the change in electrostatic charge and by superposing the oscillation caused by the bending movement. a large oscillation can be obtained from the transducer system.

When the oscillator is thick and when the direction of the oscillation caused by the bending movement of the piezoelectric polymer film based on the piezoelectric effect cannot be determined. by the phenomenon that it has previously been charged by the electrets, it

is necessary to plan the oscillator so that the direction of oscillation is predetermined. For this purpose, the oscillator film itself may be fabricated in the shape matching such a purpose. For example the oscillator film may be provided with a curvature or the oscillator film may be fixed locally at at least one position by a fixing substance at, for instance, a central portion thereof.

The piezoelectric polymer used in this invention may be a piezoelectric element made of a vinylidene fluoride resin showing large piezoelectricity. vinylidene fluoride resin in this specification refers to polyvinylidene fluoride or a copolymer of vinylidene fluoride and at least one monomer copolymerizable with vinylidene fluoride.

A non-oriented film of the vinylidene fluoride resin which was subjected to do. electric field or a nonoriented film of the vinylidene fluoride resin which was, after being uniaxially oriented subjected to a dc. electric field under heating shows a high piezoelectricity and it can be used in the electroacoustic transducer of this invention.

When an electric potential is applied to a nonoriented film of a vinylidene fluoride resin at temperatures near the melting point of the resin, the piezoelectric constant becomes higher than c. g. s. e. s. u. Furthermore, even if the heating temperature is comparatively low, a piezoelectric substance having a high piezoelectricity can be obtained by increasing the electric potential applied thereto. Such a piezoelectric polymer prepared from the non-oriented film of the vinylidene fluoride resin does not have a particularly high piezoelectricity but has the advantage that the polymer is isotropic.

On the other hand, although a piezoelectric polymer prepared from an oriented vinylidene fluoride resin film is anisotropic, the polymer can be provided with a quite high piezoelectricity. For example, by applying a high d.c. electric field to an oriented film of the vinylidene fluoride resin at temperatures of about 100C, a piezoelectric polymer film having a piezoelectric constant of about 10 c. g. s. e. s. u. can be produced. Ordinarily, by applying a electric field of more than 100 kv/cm. at temperatures between 40C. and 180C, a practically applicable piezoelectric film of the vinylidene fluoride resin can be prepared. Also, by using polymers other than the vinylidene fluoride resin, such as a vinylidene chloride resin, a vinyl fluoride resin, a vinyl chloride resin, etc., a piezoelectric polymer film showing a high stretching type piezoelectricity can be obtained and can be used in the electroacoustic transducer of this invention.

Thin small massed piezoelectric polymer films are preferable for use as a vibrator and a thin film of 2p. to 200p. in thickness is generally used. Furthermore, the gap between the piezoelectric vibrator and fixed electrode in this invention as well as condenser-type microphones is usually in a range from p. to 3 cm. It may be designed by considering only an input voltage in the case of speakers, and by considering an input acoustic wave, amplitude of vibration film, mass of vibrator, etc., for microphones.

The electroacoustic transducer of this invention has an excellent advantage in that the shape thereof may be desirably selected and further the acoustic characteristics such as frequency characteristics. directivity, etc.,

of the electroacoustic transducer can be improved quite easily. Furthermore, when such an electroacoustic transducer is utilized for headphones, speakers, etc., the thickness of the device can be greatly reduced. Further, when it is utilized as headphones, the weight of the device can also be reduced.

The equivalent circuits of the electroacoustic transducers shown in FIG. 4 and FIG. 5 are those shown in FIG. 6 and FlG. 7 respectively, that is, the circuit in which two or three capacitors have been connected in parallel. Because the deformation due to the electrostatic force is superposed by the deformation due to the piezoelectric effect, and because the capacitors are connected in parallel as stated above by referring to the equivalent circuit, the piezoelectric sensitivity of the transducer of this invention is considerably higher. Thus, it is possible to omit the transformer or reduce the winding ratio of the transformer, which makes it possible to minimize the size of the impedance transducer and to provide an electroacoustic transducer having a good S/N ratio at a low cost. Furthermore, because a transducer having a large electrostatic capacity is obtained, the influence of the distribution capacity of lead wires can be substantially reduced.

Also, the sound output from the electroacoustic transducer of this invention is sufficient for practical uses. Moreover, since the structure of the electroacoustic transducer of this invention is quite simple, the transducer can be produced easily and further the electroacoustic transducer having excellent acoustic characteristics can be produced with low cost. The electroacoustic transducers of this invention can be utilized for headphones, speakers, microphones, pickups, etc.

Now, a practical embodiment of the electroacoustic transducer of this invention composed of a combination of a piezoelectric polyvinylidene fluorde as the oscillator and an electret will be described in detail.

For providing a piezoelectricity by the application of a dc. electric field, as mentioned above, to a polyvinylidene fluoride resin showing stretching-type piezoelectricity, the following is known.

When the polyvinylidene fluoride resin film is expanded, the surface of the piezoelectric polymer film which had faced a positive electrode at the provision of the piezoelectricity is charged negatively. When the piezoelectric polymer film is contracted, the surface thereof is charged positively. Conversely, when a negative potential is applied to the surface of the piezoelectric polymer film, the polymer film contracts, while when a positive potential is applied, the polymer film expands.

Thus, in the principle of this invention, the electroacoustic transducer has a structure that the surface, of the polyvinylidene fluoride resin filmwhich had faced a positive electrode at the application of d.c. electric field for providing the piezoelectricity to the polymer film, faces, as the oscillator, the surface of an electret having positive potential.

Also, when the sign of the electric field of the electrode for applying a dc electric field at the provision of piezoelectricity to the polymer film is opposite to the sign of the charge on the surface of an electret facing the surface of the piezoelectric polymer film which had faced to the electrode, the attractive force is replaced by a repulsive force, in the same manner as above. In this case the repulsive force superposes on the expanding motion and also the attractive force superposes on the contracting motion. Thus, in this case, also, a larger deformation of the oscillator is obtained with respect to the case of employing the piezoelectric element alone. An embodiment where the piezoelectric element is always exposed to the electric field opposite to the electric field applied at the provision of the piezoelectricity to the polymer film, is preferable when the oscillator takes a large displacement or a large bending movement for, e.g., low frequency purposes, because the oscillator film is not brought into contact with the electrode even if the amplitude of the bending movement becomes larger; however, there is a fear that the internal polarization of the piezoelectric film might be destroyed to reduce the piezoelectricity. Therefore. in the case of using an electrode having a permanent polarization, such as an electret as the fixed electrode, it is preferable that the direction of the electric field of the electret be same as the direction of the electric field applied in the case of providing the piezoelectricity to the piezoelectric polymer film.

Now, the invention will be explained more practically by the following example.

EXAMPLE The polymer film for the piezoelectric element used in this example was prepared by extruding a powder of polyvinylidene fluoride produced by suspension polymerization through an extruder into a film of 100 microns in thickness and then stretching the film by a factor of four at 100C. to provide a polymer film of 25 microns in thickness.

After vacuum-depositing aluminum onto both surfaces of the polymer film, the film was placed between two electrodes and one of the electrodes was, at the positive side, connected to a high voltage generator, while the other electrode was grounded. Thus. a dc. electric field of 1000 kv/cm was applied to the electrodes at a temperature of 90C. and the polymer film was cooled to room temperature in that condition to provide a piezoelectric film to be used as the oscillator.

A sheet ofa composition of 70% by weight polyvinylidene fluoride and 30% by weight polymethyl methacylate having a diameter of 10 cm and a thickness of 1.0 mm. was placed between two electrodes and after applying a dc. electric field of 100 kv/cm. at a temperature of 90C. for 30 minutes, the polymer sheet was cooled to room temperature in that condition to provide an electret showing an electric field strength of 3000 volts/cm, which was used as the fixed electrode.

By using the oscillator and the electret prepared above, a headphone was made as shown in FIG. 8 of the accompanying drawings. In the embodiment shown in FIG. 8, the electret 10, prepared as above. has a back aluminum plate 11 vacuum coated thereon and is disposed as an oscillator facing the oscillator 12 prepared as above. The two elements are covered by protective plates 17 and 18, the protective plate 18 having perforations l9. The protective plates and the elements are separated from each other by means of annular spacers 20, 21 and 22 and the oscillator 12 is supported by means of a packing 23. The interval between the oscillator and the fixed electrode is 1 mm.

When the headphone thus prepared is connected to a headphone terminal of a commercially available record player amplifier. sufficient sound output is obtained without a bias potential circuit or transformer. in addition, in an electret-type headphone in which a polyester film having the same thickness of the piezoelectric polyvinylidene fluoride film is used in place of the piezoelectric film as the oscillator in the same structure as above, a sufficient sound output cannot be obtained without using a transformer or increasing the voltage of the input signal about 30 times by using a transformer for obtaining a sufficient sound output.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

We claim:

1. An electrostatic type electroacoustic transducer comprising:

a. oscillator means fixed at two portions thereof and including a piezoelectric polymer sheet having stretching-type piezoelectric characteristics and electrodes on both surfaces of said polymer sheet;

b. at least one fixed electrode means facing said oscillator means and spaced therefrom; and c. means for applying an alternating current electric input signal to both said oscillator and said electrode means, said oscillator means and said fixed electrode means being positioned such that. upon application of the input signal, the oscillating movement of said oscillator means is intensified by the combination of:

the bowing of said oscillator means between the two fixed portions thereof, said bowing being caused by the piezoelectric effect of the piezo electric polymer sheet; and the attractive and repulsive forces between said oscillator means and said fixed electrode means.

2. The electrostatic type electroacoustic transducer as claimed in claim 1 wherein said piezoelectric polymer sheet is a piezoelectric vinylidene fluoride resin sheet.

3. The electrostatic type electroacoustic transducer as claimed in claim 1 wherein said fixed electrode means comprises an electret having a conductor backing plate 4. The electrostatic type electroacoustic transducer as claimed in claim I wherein said fixed electrode means comprises two fixed electrodes. one on each side of said oscillator means whereby said fixed electrode means exerts a push-pull effect on said oscillator mC'tIHS. 

1. An electrostatic type electroacoustic transducer comprising: a. oscillator means fixed at two portions thereof and including a piezoelectric polymer sheet having stretching-type piezoelectric characteristics and electrodes on both surfaces of said polymer sheet; b. at least one fixed electrode means facing said oscillator means and spaced therefrom; and c. means for applying an alternating current electric input signal to both said oscillator and said electrode means, said oscillator means and said fixed electrode means being positioned such that, upon application of the input signal, the oscillating movement of said oscillator means is intensified by the combination of: the bowing of said oscillator means between the two fixed portions thereof, said bowing being caused by the piezoelectric effect of the piezoelectric polymer sheet; and the attractive and repulsive forces between said oscillator means and said fixed electrode means.
 2. The electrostatic type electroacoustic transducer as claimed in claim 1 wherein said piezoelectric polymer sheet is a piezoelectric vinylidene fluoride resin sheet.
 3. The electrostatic type electroacoustic transducer as claimed in claim 1 wherein said fixed electrode means comprises an electret having a conductor backing plate.
 4. The electrostatic type electroacoustic transducer as claimed in claim 1 wherein said fixed electrode means comprises two fixed electrodes, one on each side of said oscillator means whereby said fixed electrode means exerts a push-pull effect on said oscillator means. 